A stratospheric long endurance hybrid unmanned aerial vehicle (UAV) may perform civil duties such as meteorological observation, aerial shot, communication relay, disaster observation, Internet network construction, and the like, as well as military duties such as wider monitoring and reconnaissance at an altitude higher than that of an existing aircraft operated at the atmosphere. In addition, since the stratospheric long endurance hybrid unmanned aerial vehicle is operated at an altitude lower than that of an artificial satellite, maintenance and management of the stratospheric long endurance hybrid unmanned aerial vehicle are easy, and data loss and delay may be minimized.
Since a meteorological change is not substantially present in the stratosphere unlike the atmosphere, stability, which is one of the most importable elements in operating an aircraft, may be secured in the stratosphere. In addition, since a larger amount of solar energy may be obtained in the stratosphere than in the atmosphere, a solar cell system constituting a hybrid unmanned aerial vehicle may generate a larger amount of power in the stratosphere.
Therefore, in the case in which the unmanned aerial vehicle is operated in the stratosphere, duty efficiency may be improved, such that a service may be continuously provided.
Several study teams all over the world have developed an unmanned aerial vehicle in which a hybrid system is used. The National Aeronautics and Space Administration (NASA) has developed Pathfinder series and Helios, which are high altitude long endurance hybrid unmanned aerial vehicles, in ERAST Project up to 2003. These unmanned aerial vehicles use a hybrid system of a solar cell, a battery, and a fuel cell, and the like, to aim to be flown at an altitude of about 15 km to 30 km (50000 ft to 100000 ft) for at least twenty four hours. The Pathfinder-Plus has a total weight of 315 kg and a wing area of 87.12 m2 and has arrived at an altitude of about 24 km (80000 ft), and the Helios has a total weight of 929 kg and a wing area of 183.58 m2 and has arrived at an altitude of about 29.5 km (96863 ft).
The Thales Alenia Space has developed StratoBus, which is a high altitude long endurance hybrid unmanned aerial vehicle having an airship form. This airship uses a hybrid system of a solar cell, a battery, a proton exchange membrane fuel cell (PEMFC), and a water electrolyzer to aim to stay in the sky of 20 km for long endurance such as five years or more.
The Boeing has developed Solar Eagle capable of staying in the sky of 20 km for long endurance such as five years or more by using a hybrid system of a solar cell, a solid oxide fuel cell (SOFC), and a water electrolyzer.
The Zephyr of the Qinetiq has used a hybrid system of a solar cello and an Li—S battery to arrive at an altitude of 21.562 km on July, 2010 and achieve a long endurance record of 336 hours and 22 minutes.
However, in spite of the development of the stratospheric long endurance hybrid unmanned aerial vehicles described above, there are still issues that should be solved for the purpose of long endurance operation of the stratospheric long endurance hybrid unmanned aerial vehicles in the stratosphere.
First, it is impossible to additionally supply fuel to the unmanned aerial vehicle in the stratosphere, and in the case of using a solar cell system, power may not be generated during the night in which solar energy is not present. Therefore, an auxiliary power system is necessarily required. In addition, the stratospheric long endurance unmanned aerial vehicle should be able to ensure extreme conditions such as a very low temperature of 56.5° C. below zero, a strong ultraviolet ray, a low air density, and the like. Particularly, since lift force of the unmanned aerial vehicle is smaller in the stratosphere than in the atmosphere under a low air density condition, the unmanned aerial vehicle should be manufactured at a very light weight or a wing area of the unmanned aerial vehicle should be increased, in order to fly the unmanned aerial vehicle in the stratosphere.
To this end, in a high altitude long endurance simulation method for an unmanned aerial vehicle based on regenerative fuel cells and solar cells according to an exemplary embodiment of the present invention, a hybrid system of regenerative fuel cells and solar cells has been used in an unmanned aerial vehicle.
Hydrogen and oxygen may be generated using surplus energy remaining after a solar cell system supplies output power required by the unmanned aerial vehicle to the unmanned aerial vehicle during the daytime through a water electrolysis system of the regenerative fuel cells, and power may be generated using the hydrogen and the oxygen generated during the daytime through a fuel cell system of the regenerative fuel cells during the night in which the solar cell system may not be operated. Water generated through an electrochemical reaction in the fuel cells is used to generate the hydrogen and the oxygen through electrolysis in a water electrolysis stack. Therefore, the unmanned aerial vehicle using a hybrid system of the regenerative fuel cells and the solar cells according to an exemplary embodiment of the present invention does not require the additional supply of fuel, such that it may be a good solution for solving the problems described above.
In addition, specific energy of the regenerative fuel cell system is 400 to 1000 Wh/kg, which is higher than 240 Wh/kg corresponding to specific energy of a lithium-ion battery system. This means that the regenerative fuel cell system is lighter than the lithium-ion battery system in providing the same energy.
Therefore, in the case in which the regenerative fuel cells are used in the unmanned aerial vehicle, the unmanned aerial vehicle may be manufactured at a weight lighter than that of an unmanned aerial vehicle using a battery.
However, in a regenerative fuel cell system of a current technical level, a technical difficulty for manufacturing the regenerative fuel cell system at a very light weight is high, and hydrogen/oxygen generation efficiency of a water electrolyzer is low, such that it is very difficult to achieve a long endurance duty.
Therefore, in the high altitude long endurance simulation method for an unmanned aerial vehicle based on regenerative fuel cells and solar cells according to an exemplary embodiment of the present invention, in order to search a target value of a technical level for achieving a high altitude long endurance target of the unmanned aerial vehicle, specific design variables having an influence on endurance capacitance of the unmanned aerial vehicle are set, and a simulation is performed while controlling the respective specific design variables to analyze influences of the design variables on a flight time, thereby making it possible to provide target values of the design variables required for a long endurance operation.
Meanwhile, Korean Patent Laid-Open Publication No. 10-2012-0109563 (entitled “High Altitude Long Endurance Unmanned Aircraft and Methods of Operation Thereof” and hereinafter referred to as Related Art Document 1) has disclosed a high altitude long endurance unmanned aerial vehicle having high altitude base station maintaining capability, but does not mention the problems described above and a method for solving the problems described above at all.